The present invention relates to a method of controlling a filling process for injection molding machines, and a control device therefor.
An explanation will be given to a molding operation of an injection molding machine. In particular, an explanation will be given to the case of a motor driven injection molding machine to center on motions of an injection device.
(A) A servo-motor for driving rotation of a screw rotates the screw whereby resin falling on a rear portion of the screw from a hopper is melted and a given amount thereof is fed to a tip end of a heating cylinder. At this time, the screw retreats while being subjected to pressure of molten resin accumulating at the tip end of the heating cylinder.
Connected directly to the rear end of the screw is a drive shaft.
The drive shaft is rotatably supported on a pressure plate through bearings. The drive shaft is driven through a timing belt by a servo-motor for driving rotation of the screw. The pressure plate is driven through a ball screw by a servo-motor for injection to advance and retreat along guide bars. The foregoing pressure of molten resin is detected by a load cell in a manner described later. A detected value of the load cell is fed back by a feed-back control loop for pressures.
(B) Then driving of the servo-motor for injection causes the pressure plate to advance to fill molten resin into a metal mold with the screw tip end as a piston.
(C) At the end of the filling process, the molten resin fills a cavity of the metal mold. At that time, the advancing motion of the screw causes conversion of velocity control into pressure control. Such conversion of velocity control into pressure control is called a V-P conversion.
(D) Thereafter, the resin in the cavity of the metal mold becomes cold under a set pressure. Resin pressure is controlled in feed-back control loop like the above-mentioned pressure control.
In the injection device, when the process (D) is terminated, it goes back to the process (A) and shifts to the succeeding molding cycle. Meanwhile, in a mold clamping device, concurrently with the process (A), the metal mold is opened to permit an ejector mechanism to discharge a molding product having been cooled and solidified, and then the metal mold is closed to shift to the process (B).
With reference to FIG. 1, an explanation will be given hereinbelow to the molding motion of a motor driven injection molding machine to center on motions of an injection device. The injection device performs filling of molten resin by converting rotating motion of a servo-motor into linear motion with the use of a ball screw and a nut. In FIG. 1, rotation of the servo-motor 10 for injection is transmitted to a ball screw 11. A nut 12 adapted to advance and retreat upon rotation of the ball screw 11 is fixed to a pressure plate 13. The pressure plate 13 is movable along a plurality of guide bars 14 (only two being shown) fixed to a base frame (not shown). Advancing and retreating movements are transmitted to a screw 18 through a load cell 15, a bearing 16, and a drive shaft 17. The drive shaft 17 is also rotatingly driven through a timing belt 20 by a servo-motor 19 for driving rotation of the screw.
Rotating driving of the servo-motor 19 causes the screw 18 to retreat in a heating cylinder 21 while rotating whereby molten resin is accumulated at the tip end of the heating cylinder 21. And rotating driving of the servo-motor 10 causes advancement of the screw 18 to thereby fill the metal mold with the accumulated, molten resin and pressurize the resin for molding. At this time, forces, which push the resin, are detected as reaction forces by the load cell 15.
A detected value from the load cell 15 is amplified by a load cell amplifier 22 to be input into a controller 23. Mounted on the pressure plate 13 is a position detector 24 for detection of amounts of movements of the screw 18. A detected value from the position detector 24 is amplified by an amplifier 25 to be input into the controller 23.
In accordance with setting established by an operator, the controller 23 outputs to servo-amplifiers 26, 27 current (torque) commands depending upon the respective processes. The servo-amplifiers 26, 27 control drive currents of the servo-motors 10, 19 to control output torque of the motors.
In the above-mentioned injection device, pressure control is conventionally carried out after V-P conversion. Pressure control is slow in response speed. As a result, this is responsible for dispersion in weight of molding products and over-filling among molding products, for which rapid depressurization from the filling pressure to the dwelling pressure is required.
FIG. 2 shows an example of varying waveforms of filling velocity and of resin pressure before and after the V-P conversion. As apparent from FIG. 2, with the prior art, rapid depressurization is difficult due to slow response speed since it is switched to pressure control after the V-P conversion. For example, with molding products for DVD (Digital Video Disc) and connectors, a dwelling process is very short. In this case, the manner of depressurization after the V-P conversion affects a molding product much. On the other hand, with molding products, which require high injection velocities, pressure changes very fast, so that pressure cannot be controlled by the response of pressure control.
It is an object of the present invention to provide an injection molding method, which can achieve stability in quality of molding products by retreating a screw immediately before conversion from velocity control into pressure control to obtain a required pressure waveform.
It is another object of the invention to provide a control system suited for the above-mentioned method.
An injection molding method according to the present invention comprises the steps of melting molten resin within a heating cylinder, advancing an injection screw within the heating cylinder to inject the molten resin into a mold, retracting the injection screw to a predetermined position before completion of the injection, further advancing the injection screw under pressure control.
An injection molding machine control system according to the present invention comprises a velocity control system which comprises a position detecting unit for detecting a position of an injections screw, a position setting unit for providing a set value of a position of the injection screw, and a first feedback circuit for outputting a first speed command value to an injection motor based upon a difference between a detected value from the position detecting unit and the set value from the position setting unit. The system also comprises a pressure control system which comprises a pressure detecting unit for detecting a resin filing pressure, and a pressure setting unit for providing a set value of a filling pressure and a dwelling pressure, and a second feedback circuit for outputting a second speed command value to the injection motor based upon a difference between a detected value from the pressure detecting unit and the set value from the pressure setting unit. The system further comprises a switching unit for switching between an output of the first feedback circuit and second feed back circuit to provide a selected speed command value to the injection motor. The velocity control system is configured to perform a control action for returning the injection screw to a set at a predetermined velocity when the injection screw has advanced to a predetermined position during a filing process, and the switching unit switches to the second feedback circuit thereafter.